The aim of this psychophysical project is to study the expression, and the neuronal mechanisms underlying this expression, of two forms of human visual attention: focal or serial attention and a saliency-based form of visual attention. This latter form of attention is believed to operate in parallel throughout the entire visual field and is based on center-surround type of neuronal operations. Preliminary psychophysical evidence supports two such forms of complementary attentional processes that can operate independently of each other. To further distinguish these two different attentional processes operating in the primate visual system, this study plans to carry out three sets of psychophysical experiments (using computer generated and controlled stimuli displayed on a monitor). The experiments address the question of whether there only exists a single form of focal attention, to what extent both forms of attention are influenced by prior visual experience on the subject and which of these two forms are necessary for conveying precise spatial relationships (e.g. the triangle is to the left of the circle). Computer modeling will be used to complement the experimental program. The scene will be represented in a large number of cortical maps, representing different features, such as intensity, color (red-green and blue-yellow), different orientations and motion. Neurons in these maps will have receptive fields operating at many different spatial scales and will interact with each other in accordance with the known physiology of the visual system of macaque monkeys and humans. The output of these neurons will feed into a single saliency map that expresses the conspicuity or saliency of particular features in images. This computer model will be used to replicate and predict psycho-physical performance.